The present invention concerns the field of catalysis, particularly the field of heterogeneous catalysis.
Its object is a catalysis system based on metallic oxides of the MO2 type.
Its object is also the process of obtaining catalytic systems as well as the application of those systems in the chemical industry, especially in petro-chemistry, for the reactions of isomerization, dehydrogenation, hydrogenation and/or in hydrogenolysis of saturated and/or unsaturated organic compounds.
In the chemical industry, most reactions are accelerated by catalysts whose function is to allow the progress of those reactions under conditions of temperature and pressure which are economically profitable.
We already know four major types of catalysts in the field of heterogeneous catalysis or (contact catalysis):
The pure metals(4e, 5e, and the 6e periods of transition metals)
The oxides of transition metals or of some heavy elements showing several stable oxidation states.
The solid oxides of metals from the two first columns IA and IIA of the periodic table.
The light metals and non-metals from columns IB, IVB, VB (acid oxides)
Within those different categories, we can distinguish two sub-categories according to their way of preparing this system: Bulk catalyst and supported catalyst, i.e. whose active phase is deposited on a support.
In the chemical industry in general, acid oxides (alumina, silica, ofen mixed, zeolites) catalyse mainly the hydration, isomerization, alkylation and cracking of organic molecules.
Certain oxides can catalyse at the same time redox reactions and acido- basic reactions: these are called bifunctional catalysts (reformation of fuels, synthesis of butadiene).
In the petrochemical industry in particular, the use of catalysts in the isomerization process, allows the obtaining of hydrocarbons with high octane number which can be used directly.
However, the catalysts used nowadays for the types of reactions mentioned above, still present a lot of inconveniences, some of which are important.
Indeed, a big part of the known catalysts contain noble metals such as platinum, paladium or iridium. The content, even if.very small, of such metals in the known catalysts, as well as the difficulties in recycling them, explain the very high prices of such systems.
Furthermore, research trying to replace those noble metals by cheaper metals in order to obtain new efficient catalytic compounds has not brought any really satisfying solutions up to now.
In particular, supported metallic catalysts, well-known for their activity in terms of hydrogenolysis and isomenzation, have been the subject of studies on the substitution of noble metals by oxides of transition metals in particular.
However, the physico-chemical performances in terms of conversion, selectivity, life-time and recycling capability of the catalysts proposed at the end of these studies, are not always up to the industrial expectations, which is all the more prejudicial, as a good number of those new catalysts are often usable for a limited number of compounds and for specific reactions. Moreover, the development of a catalyst with the exact objective is generally uncertain, long and expensive.
As far as the bifunctional catalysts are concerned, one has to note that the acid character is brought by the support, which nowadays is usually an acid or chlorated alumina, eventually a zeolithe, whereas the metallic character is brought by a deposited metal. The necessary presence of two active substances leads to problems too, which are all the more important as these active substances are different, problems such as manufacturing, high costs, incompatibility between the materials and their treatment.
The extensive hydrogenolysis properties of tungsten and molybedenum carbides have been clearly demonstrated by A. Katrib et al. Cat. Lett., 38(1996)95, and it has been shown that the presence of oxygen leads to the formation of oxycarbides WOxCy type compounds which provide the isomerization catalytic properties to these new systems.
An identification, in particular by X-ray photoelectron spectroscopy of these new systems enabled to identify the active species as WO2 and MoO2 with isomerization properties (A. Katrib et al. J. Electron. Spectro. Relat. Phenomenon. 76(1995)195, and J. Chim. Phys. 94(1997)1923). On the other hand, the existence of the oxycarbide species WOxCy has been excluded. Also, it has been shown that the W or Mo pure metals have hydrogenolysis properties, whereas the WO3 and MoO3 trioxides are catalytically inactive concerning saturated hydrocarbos.
Certain research on these trioxides deposited on alumina(W. Grxc3xcnert et al. J. Cat. 107(1987)522) have shown that the support stabilises the oxide WO3 in terms of a strong metal-support interaction. It is therefore difficult to form WO2, which is responsible for the catalytic activity on such a support.
In the isomeriztion catalysts, we can refer to the works of Martin (C. Martin et al. Cat. Lett. 49(1997)235) which describes the overall technical experiments allowing to characterize this type of catalysts. Moreover. Vermaire and Van Berge (J. Cat. 116(1989)309) have directed their work on the preparation of isomedzation catalysts, emphasizing among others, the influence of the pH, which is less important in the case of TiO2 than on Al2O3. They also proposed a mehanism allowing to interprete the stoichiometric ratio 1:1 during the adsorption of the WO3 on the sites Tixe2x80x94Oxe2x80x94Ti type, and they have shown the importance of a monolayer of WO3 deposited on TiO2. According to them, this one corresponds to the maximum amount of tungsten which can accurnaate on TiO2 when it is placed for impregnation in a solution of pH=2. However, Rondon, Howalla and Herculs have established that this maximum quantity depends on the pH of the impregnation solution (Surf. Interface Anal. 26(1998)329).
The works of Yamaguchi, Tanaka and Tanabe (J. Cat. 65(1980) 442) have shown that the activity of catalysts based on tungsten studied in their work, reached a limit value for an initial content of WO3 of 8% (molar). This quantity corresponds in fact to the triple of the formerly identified monolayer. They have also correlated activity to the acidity which occurs during the mixing of the two oxides TiO2 and WO3. This acidity of Lewis type is interpreted by the accumulation of positive electrical charges on the tungsten according to the theory established by Tanabe et al. (Bull. Chem. soc. Japan 47(1974)1064). It is then possible, in the presence of water, to obtain the Brxc3x6nsted acidity.
Finally, Hino and Arata (Bull. Chem. soc. Japan 67(1994)1472), have prepared solid superacid, by impregnating titanium hydroxide with WO3. It is not really a question of deposition of WO3 on a suport, since the support (TiO2) is obtained by calcining the corresponding hydroxide after impregnation of the tungsten species. The application of these catalysts for the reactions of isomerization has not been considered.
The problem to be solved by the present invention consists therefore in supplying a bifunctional catalyst, which is cheap and stable in function of time, polyvalent and performing.
To this purpose, its object is a polyvalent bifunctional catalyst, characterized by the presence of MO2 type phase, supported on TiO2. This MO2 phase is obtained by the reduction of the corresponding MO3 oxide(s). The metal(s) forming the MO2 oxides are preferrably chosen in the group formed by W and Mo.